Method and apparatus for lowering the chlorate content of alkali metal hydroxides



METHOD AND APPARATUS FOR LOWERING THE CHLORATE CONTENT OF DROXIDESSidney G. Osborne, Saint Davids, Ontario, Canada, as-

signor to Hooker Electrochemical Company, Niagara Falls, N Y., acorporation of New York ALKALI METAL HY Serial No. 403,923

9 Claims. l. 204-98) No Drawing. Application January 1954 This inventionrelates to methods and. apparatusfor reducing the amountof chlorates incaustic solutions produced by electrolysis in ordinary diaphragm typecells.

In the electrolytic cells of the diaphragm type, such as the type ofcell described in United States Patent 1,866,065, the anode compartmentis separated from the cathode compartment by a permeable diaphragm.Alkali metal chloride brine, such as lithium, sodium, or potassiumchloride, is introduced. into the anode compartment, where it comes intocontact with the anodes, and is caused to percolate through thediaphragm into the cathode compartment, where it comes into contact withthe cathodes. When an electric current is passed between theseelectrodes, chlorine is liberated at the anodes and alkali metalhydroxide is formed at the cathodes with the liberation of hydrogen. Inorder to minimize voltage drop in the cell, the cathodes are placed ascl-ose to the A'review of the electrochemistry of deposited diav phra'gmtype cells has been given by Murray, R." L.', and

Kircher, M. S. in the Transactions of the Electrochemical Society; vol.86, pp. 83-106; 1944.

Chlorine, as such and as hypochlorous acid, is more or less soluble inbrine, even at elevated temperatures, and forms hypochlorites inaccordance with the followin representative equations:

' Thus, some chlorine inevitably passes through the diaphragm insolution in the percolating brine. When coming into contact with thecaustic alkali in the cathode compartment, this chlorine reacts with thealkali to form alkali metal hypochlorite, in accordance with thefollowing representative equations:

ing oxygen. However, at normal an yte P is 2,823,??? r mmed Feb. 11,1958 about 4, hypochlorous acid may be formed by reaction with thechlorine in accordance with the following equa- Furthermore; since thehydroxyl ions carry a negative charge, which is discharged at the anode,their back migra tion represents a loss in current efiiciency.

Hypochlorite ion, which'is formed from the hydrolysis of chlorinedissolved in the anolyte, is discharged, at the anode to form chlorateion in a manner after the followingequationz 5 1 12C1O- i.6H O- 4ClO3--[-8Cl"+ 12H++ 3O +12e (7) Further, hypochlorous acid andhypochlorite ion are unstable under the, conditions of electrolysis andtend to form, chlorate ion' and oxygen according to the followingequations:', 5' ,7 .:-ClO.*+2HClO.-- ClO -+2Cl-+2I-I+ (8) The oxygenproduced from hydroxyl ions discharging at the anode and from thedecomposition of some of the hypochlorites in the anolyte therebyresults in contaminationbf the chlorine; also, since the anodes are ofgraphite, some of the oxygen attacks the anodes, slowly consuming them,which results in the contamination of the chlorine with .carbon dioxide.Similarly, the oxygen produced from the decomposition of some of thehypochlorites in the catholyte results in contamination of the hydrogenwith oxygen. 7 I v In the cathode compartment substantial amounts .ofthe hypochlorite and chlorate ions are reduced by nascent hydrogen (Hformed at the cathode according to the following equations:

However, some of the hypochlorite and chlorate ions escape reduction inthe catholyte and pass out of the cell and thereby contaminate the celleffiuent which is mainly spent brine having the alkali metal;hydroxidedissolved therein. g a g In the presence of an excess of alkali, thechlorate is quite stable. It therefore tends to persist in the celleffluent and to pass on through to the evaporators in which the causticalkali is concentrated. Practically all of the chlorate survives theevaporation and remains in the final product, where it constitutes ahighly objectionable contaminant,especially to the Rayon industry.

The problem of lowering chlorates has been attacked at two main points:p I a (a) The chl-orates having been formed, can be reduced in thefurther processing of the caustic alkali and by special treatingmethods. See for instance, U. S. Patents 2,622,009; 2,044,888;2,142,670; 2,207,595; 2,258,545; 2,403,789; 2,415,798; 2,446,868; and2,562,169; and British patents 642,946 and664,023 which showrepresentative examples of different methods used for reducing thechlorates after they have been formed.

(b) The productionof chlorates during the electrolysis can be lowered byadding a reagent to the brine feed which reacts preferentially withztheback'migrating hydroxyl ions from thelcathode compartment of the cellmaking their way through the diaphragm into the anode compartment,and'by such a reaction prevents the formation of some of thehypochlorites in the manner shown by Equation 6 and thus additionallypreventing these hypochlorites from further. reacting to form chloratesin the manner shown by Equations -7, 8, and 9. Reagents such ashydrochloric 'acid shown in -U. S. Patent 583,330, and sulfur in an9xidiza l form, su has sodium tetrasulfide, shown in 3 U. S. Patent2,569,329 are illustrative of methods which have been used to attacktheproblem of chlorates in caustic by removing the back migrating hydroxylions before they can react to form chlorates.

It is an object of this invention to provide a new point of attack forthe lowering of chlorates in caustic alkali solutions produced in cellsof the diaphragm type. Another object is to provide a method for theprevention of chlorate formation during the electrolytic decompositionof alkali metal chlorides in diaphragm type cells wherein thehypochlorites are reduced before they can form chlorates. A third objectof this invention is to provide a method for the prevention of theformation of chlorates during the electrolysis of alkali metal chlorideswhich is economical to adapt into the electrolysis process and does notintroduce into the cell effiuent any undesirable impurities. A fourthobject of this invention is to provide an improved diaphragm for theelectrolysis of alkali metal chlorides. A fifth object is to provide amethod for preventing the formation of chlorates during the electrolysisof alkali metal chlorides in cells of the diaphragm type, which methodcan be used in conjunction with other known methods for loweringchlorates to give an improved result thereover, and without any harmfuleffects therewith.

I have discovered that the problem of lowering chlorates can be attackedat another point. My discovery is that production of chlorates duringelectrolysis of alkali metal chlorides in diaphragm type cells can beprevented by attacking the overall reaction at the point of hypochloriteformation, that is, destroying the hypochlorite before it can react toform chlorate.

I have now found that if small amounts of a finely divided materialselected from the group consisting of nickel, cobalt, their brinesoluble salts, their hydroxides and mixtures thereof are dispersed intothe diaphragm, and the resulting diaphragm containing the finely dividedcompounds used in the electrolysis of alkali metal chlorides, thechlorate content of the resultant cell effiuent can be loweredsubstantially from what the chlorate content would have been had such acompound not been added; likewise, the-chlorate content of the alkalimetal hydroxide produced therefrom is proportionately lowcred.

Although I do not wish to be limited to any-theory, I believe thereduction of the hypochlorites might be effected in a manner after thefollowing illustrative equa- .tions:

The reduction of the hypochlorites might also be effected in a mannerafter Equation above to a greater extent than if the finely dividedcompound of my invention had not been added to the diaphragm. I

The compounds to be used in this invention are powdered or otherwisefinely divided nickel and cobalt, and their brine soluble salts ofnickel and cobalt which upon exposure to dilute caustic solutions suchas are found in diaphragm cells during the electrolysis of alkali metalchlorides, react to form insoluble hydroxide salts of nickel and cobaltand are thereby retained in the diaphragm and not dissolved away duringcontinuous use of the diaphragm in the electrolysis of alkali metalchlorides. Mixtures of nickel, cobalt, their brine soluble salts andtheir hydroxides thereof may also be used herein. For instance, duringelectrolysis the catalytic materials dispersed into the diaphragm may beboth finely divided nickel and finely divided hydroxides of nickel; or,the materials may be powdered cobalt and finely divided hydroxides ofcobalt; or they may be powdered nickel and cobalt; or, they may bepowdered nickel and powdered cobalt and the finely divided hydroxides ofnickel and cobalt; or, the'catalytic material may be the finely divided'4 hydroxides of nickel and cobalt. I prefer to use nickelin a finelydivided state, such as powdered nickel commonly avail-able in saleableform, or a soluble salt of nickel such as a chloride of nickel which isalso commonly available. However, finely divided cobalt and solublesalts thereof such as cobalt chloride, are also satisfactory. Otherbrine soluble salts of nickel and cobalt or mixtures thereof which maybe used to disperse the catalyst into the diaphragm are their acetates,ammonium chlorides, ammonium sulfates, ammonium nitrates, bromides,ammonium bromides, chlorates, fluorides, formates, hypophosphite,iodides, nitrates, perchlorates, sulfates and others. The particularbrine soluble salt used is not important, because the catalyst isconverted over into an insoluble hydroxide form during'electrolysis.

The amount of catalyst to be dispersed into the diaphragm may be variedwithin wide ranges without depart- .ing from the scope of thisinvention. In general, a catalytic. amount is all that is necessary. Ihave found that catalyst concentrations in the diaphragm as low as 0.5percent by weight and as high as 10 percent by weight of the diaphragmare effective. Concentrations as low as 0.25 percent and as high as 20percent may also be used,

but catalyst concentrations above 20 percent are not necessary and maybe uneconomical to use. I prefer to use a catalyst concentration ofbetween one percent and 5 percent by weight of the diaphragm.

The diaphragms of this invention may be fabricated by employingpermeable diaphragm material in any of the usual'forms. Asbestos is themost commonly used material for preparing permeable diaphragms and anyof the commercially available forms may be used, such as asbestos paper,asbestos cloth, finely divided asbestos and other forms of asbestos mayall be included within the scope of this invention as being adaptable tofabrideposited diaphragm having dispersed thereon a minor amount ofcatalyst which promotes the reduction of the hypochlorites as they comeinto association therewith. Similarly, the catalysts may be dispersedinto other types of diaphragms, such as asbestos paper and asbestoscloth type diaphragms, by dispersing minor amounts of the materials intofinely divided or porous materials. For example, where possible I preferto disperse the catalytic material into the diaphragm while thediaphragm is being fabricated, in order to obtain a more uniformdistribution of catalyst throughout the porous surfaces of the resultingimproveddiaphragm.

After fabricating the improved diaphragms of this invention and whileunder the influence of the electrochemical conditions existingrwithindiaphragm type cells during the electrolysis of alkali'metal chlorides,the brine soluble salts of cobalt and nickel are converted over toabrine insoluble hydroxide form; Thus, during electrolysis the improveddiaphragms of this invention have dispersed therein a catalytic amountof a material selected from the group consisting of nickel, cobalt,their hydroxides, their brine soluble salts, and mixtures thereof.

My invention is more fully described by the following examples but I donot wish to be limited thereto except as defined inthe appended claims.

EXAMPLEI Part1 Adeposited diaphragm was fabricated a slurry of asbestossuspended in brine through a steel screen of the type used as cathodesin cells for the electrolysis of alkali metal chlorides. After most ofthe brine had drained through the screen, the mat of asbestos remainingon the screen was drawn down onto the screen more firmly by means of amild vacuum being placed on the back side of the screen. This depositeddiaphragm was assembled into a horizontal cell, constructed essentiallyof a steam-heated Biichner funnel into which was placed the steelcathode screen having the asbestos diaphragm deposited thereon and abovethe diaphragm was suspended a horizontal type graphite anode. Brine wasfed in from the anode side and the cell brought to about 84 degreescentigrade by passing steam into the jacket of the Biichner funnel,electrical connections were made between the graphite anode and steelcathode screen, and the current turned on and regulated to about 8.0amperes. The cell was operated in this manner with the brine feedinginto the anode compartment and exiting from the cathode compartment outof the bottom of the Btichner funnel at the rate of about 1.7 cubiccentimeters of flow per minute for a period of about two hours at whichtime it was observed that equilibrium operating conditions had beenreached. The sample of cell effluent taken at this time analyzed 144grams per liter of sodium hydroxide and 1.89 grams of sodium chlorateper 1000 grams of sodium hydroxide.

Part 2 A second deposited diaphragm was fabricated as in Example I, Part1, above, except that the slurry of asbestos suspended in brine alsocontained nickel powder in an amount such that the asbestos deposited onthe steel screen contained percent by weight nickel dispersed therein.The diaphragm thus formed was placed in the same cell of Example I, Part1, and operated in substantially the same manner. The sample of the cellefiluent taken after equilibrium operating conditions had been reached,analyzed 148 grams per liter of sodium hydroxide and 0.17 gram of sodiumchlorate per 1000 grams of sodium hydroxide.

A comparison of the results of Part 1 with the results of Part 2 of thisexample shows that the chlorate content of the cell effluent was reduced91 percent by dispersing 10 percent powdered nickel into the depositeddiaphragm of the cell.

EXAMPLE II The following table gives the results obtained in theoperation of commercial cells of the type illustrated in U. S. PatentNo. 1,866,065, with brine saturated at 60 degrees centigrade andcontaining about 310 grams of sodium chloride per liter and into thedeposited diaphragms of which have been dispersed varying amounts ofpowdered nickel. The percentages of nickel given are based on thecombined dry weight of asbestos and nickel deposited. The chloratecontents are the averages of several months operation in each of theindividual No harmful or abnormal effects were observed during theperiod that each of the above cells was run.

I have also found that my method for preventing chlorate formationduring the electrolysis of alkali metal chlorides in deposited diaphragmtype cells can be used in conjunction with other known methods forreducing chlorates duringelectrolysis such as the method given in U. S.Patent 2,569,329, which includes adding a reagent to the brine feedwhich reacts preferentially with the back migrating hydroxyl ions, andthat no harmful effects are found due to this joint process. Further, Ihave found that these two methods have an additive effect on the actualoverall reduction in chlorate content of the cell efiluent. Thesefindings are illustrated in Example III.

EXAMPLE III A commercial cell of the type illustrated in U. S. PatentNo. 1,866,065 having a deposited diaphragm in which was dispersedtherein finely divided nickel chloride in the amount of 2.4 percent byweight was operated in the same manner as those cells shown in ExampleII except that the brine feed contained 0.03 weight percent sodiumtetrasulfide. The average chlorate content over a period of severalmonths operations was 0.28 pound of sodium chlorate per 1000 pounds ofsodium hydroxide. No harmful or unusual efiects were observed duringthis period.

Thus in a manner after Example III the method of this invention may beused in conjunction with the prior art methods of adding an acid reagentsuch as hydrochloric acid or a reagent of the type described in U. S.Patent No. 2,569,329, that is, a sulfur compound in an oxidizable state,to the brine feed to help lower the chlorate formation even more than itwould have been lowered had my method not been used with one of theseknown methods.

I claim:

1. In the method of decomposing alkali metal chlorides in electrolyticalkali chlorine cells of the diaphragm type, the improvement whichcomprises effecting said electrolysis in the presence of a finelydivided catalytic material selected from the group consisting of nickel,cobalt, their hydroxides and mixtures thereof dispersed in saiddiaphragm wherein said finely divided material dispersed in thediaphragm is between about 0.25 percent and about 10 percent by weightof the diaphragm.

2. The method of claim 1 wherein the amount of material dispersed in thediaphragm is between about 1.0 percent and about 5.0 percent by weightof the diaphragm.

3. The method of claim 1 wherein the material is powdered nickel.

4. The method of claim 1 wherein the material is an hydroxide of nickel.

5. In a method for decomposing sodium chloride brine in electrolyticalkali chlorine cells of the diaphragm type, the improvement whichcomprises: effecting said electrolysis in the presence of a finelydivided hydroxide of nickel dispersed in said diaphragm in an amountbetween about 0.5 percent and 10 percent by Weight of the diaphragm.

6. A diaphragm for use in electrolytic cells for the electrolyticdecomposition of alkali metal chlorides comprising a permeable asbestosdiaphragm having dispersed in finely divided form therein a catalyticmaterial selected from the group consisting of nickel, cobalt, theirhydroxides and mixtures thereof wherein said finely divided materialdispersed in the diaphragm is between about 0.25 percent and about l0percent by weight of the diaphragm.

7. A diaphragm according to claim 6 wherein the asbestos is deposited ona steel screen.

8. The diaphragm of claim 7 wherein the catalytic material is powderednickel.

9. The diaphragm of claim 7 wherein the catalytic material is anhydroxide of nickel.

(References on following page) 7 References Cited in the file of thispatent 2,031,844 UNITED STATES PATENTS 368,608 Peyrusson Aug. 23, 1887389,186 Askew et a1. Sept. 11, 1888 669,441 Frasch Mar; 5, 1901 797,5471,826,724 BOOSS Oct. 13, 1931 {16,614

' 8 Moore, Feb. 25, 1936 Ove'r dick et a1 June 23, 1936 wwofodbridgeDec. 7, 1937 FOREIGN PATENTS Fral lce Apr. 28, 1936 Norway June 24, 1929

1. IN THE METHOD OF DECOMPOSING ALKALI METAL CHLORIDES IN ELECTROLYTICALKALI CHLORINE CELLS OF THE DIAPHRAGM TYPE, THE IMPROVEMENT WHICHCOMPRISES EFFECTING SAID ELECTROLYSIS IN THE PRESENCE OF A FINELYDIVIDED CATALYTIC MATERIAL SELECTED FROM THE GROUP CONSISTING OF NICKEL,COBALT, THEIR HYDROXIDES AND MIXTURES THEREOF DISPERSED IN SAIDDIAPHGRAM WHEREIN SAID FINELY DIVIDED MATERIAL DISPERSED IN THEDIAPHRAGM IS BETWEEN ABOUT 0.25 PERCENT AND ABOUT 10 PERCENT BY WEIGHTOF THE DIAPHRAGM.